A method to predict the fatigue life and the residual strength of composite materials subjected to variable amplitude (VA) loadings

A method to predict the fatigue life and residual strength of fiber reinforced composites subjected to variable amplitude (VA) loadings was developed. Based on a recent two-parameter residual strength model for constant amplitude (CA) loadings and coupled with the equivalent residual strength assumption (ERSA) the method allows accounting for the cycle-by-cycle loading variability and to build on a solid foundation a numerical procedure for fatigue calculations under spectrum loadings. The responses of quasi-isotropic carbon/epoxy laminate subjected to ordered and random block loadings were first tested and the simulations revealed that the load order strongly impacts residual strength. Then the approach is extended to a large database available in literature concerning the residual strength of E-glass/vinyl ester composite laminate subjected to ordered ascending (L-H) and descending (H-L) and random block spectra using a reference spectrum with of 735,641 cycle and 22 stress levels. It is shown that the predictions agree very well with experimental results including a series of premature failures that remained unpredicted before this study.